This invention relates to an activated sludge sewage treatment process for treating raw or treated phosphate-containing sewage to obtain a substantially phosphorous-free effluent which is returned to natural water resources.
In the conventional activated sludge system in use today, sewage is subjected to the usual screening and preliminary sedimentation procedures, then mixed with activated sludge recycled from a settling tank to form a mixed liquor which is subjected to aeration. During aeration of the mixed liquor, the organisms present cause the aerobic decomposition of solids and a high degree of BOD removal is achieved.
Phosphates, which are present in organic wastes and detergents, escape conventional sewage treatment processes and are released with the effluent into natural water resources, e.g., lakes, rivers and streams. These phosphates result in over fertilization or eutrophication of waters causing unsightly algal blooms and serious pollution problems.
It is known that aeration of the mixed liquor in an activated sludge sewage treatment process initially causes the microorganisms present to take up phosphate. U.S. Pat. No. 3,236,766 discloses a process which utilizes this phenomenon for removing phosphates from sewage. According to the process disclosed in that patent, the pH of raw sewage is adjusted, if necessary, to maintain a range of from about 6.2 to about 8.5, the sewage is mixed with activated sludge to form a mixed liquor, the mixed liquor is aerated to maintain a dissolved oxygen content of at least 0.3 mg. per liter in the mixed liquor and a phosphate-enriched sludge is separated from the mixed liquor to provide a substantially phosphate-free effluent. The phosphate-enriched sludge is treated to reduce the phosphate content thereof prior to recycling for mixing with the influent sewage. This is accomplished by maintaining the phosphate-enriched sludge in an anaerobic condition for several hours.
Several other processes have been proposed for reducing the phosphate content of phosphate-enriched sludge following the aeration step in an activated sludge sewage treatment process. Thus, for example, U.S. Pat. No. 3,385,785 discloses adjusting the pH of phosphate-enriched sludge to between about 3.5 and 6 and agitating the sludge in contact with a low phosphate-containing aqueous medium in a tank for a time sufficient to effect transfer of water-soluble phosphate material from the sludge to the aqueous phase. The low phosphate-containing aqueous medium used in this tank is water added from an external source to leach out the phosphate from the sludge. After the soluble phosphate has been leached out of the sludge into the aqueous medium, the mixture is passed to a settling tank wherein the phosphate-enriched aqueous medium is separated from the phosphate-depleted sludge. Since this process requires the addition of considerable quantities of water from an external source and requires two separate tanks -- i.e., a phosphate leaching tank and a settling tank, to separate the soluble phosphate from the sludge, a large capital investment is required in such a process.
Accordingly, it is an object of this invention to provide an improved process for reducing the phosphate content of phosphate-containing sewage in an activated sludge sewage treatment process.
It is a further object of this invention to provide such a process which includes an improved phosphate stripping operation of phosphorous-enriched sludge and which results in increased efficiency of the phosphate removal from the sewage.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and claims.